More and more conventional procedures, in which the spine is exposed in the area in which the surgery is to be performed by considerable incisions in the back of the patient and by cutting through muscles, are replaced in spinal surgery by percutaneous spinal surgery. To make it possible to find ways for inserting instruments through the skin as well as through tissues located in front of the spine and the vertebrae as well as to make it possible to work percutaneously with such devices, a guide sleeve or working sleeve having a sufficient lumen and hence a sufficient cross section is inserted. This cannot be easily inserted as such. The skin and the subjacent tissue is rather pierced first with a thin needle kit comprising a stylet and a hollow needle narrowly surrounding same, both of which taper distally, i.e., are pointed (Jamshidi needle). The stylet is then removed from the hollow needle, and a guide wire is passed through the hollow needle to the intervertebral disk space or intervertebral foramen. The hollow needle is then removed and a first dilator is inserted via the guide wire. Additional dilators, which are closely adapted with their lumen to the outer contour of the dilator inserted previously, are inserted via this dilator. The dilators are tapered conically at their ends. This is carried out until the guide sleeve or working sleeve can then be inserted via a last dilator, and the guide wire and the dilators are then removed from this guide sleeve or working sleeve, and the surgeon can then perform the procedure through the lumen by inserting instruments, etc., and via these instruments or by inserting an intervertebral cage (interbody cage) into the intervertebral foramen. All the above-mentioned steps as well as the further steps are carried out under X-ray monitoring.
The dilators and especially the working sleeve or guide sleeve have hitherto regularly been made with a cylindrical configuration. It was found that especially when inserting an interbody cage, the stress of the patient, which is substantially reduced by the percutaneous procedure only compared to an open surgery, is still undesirable, because especially the lateral cross-sectional dimensions of a cage are rather large and the diameter of a conventional cylindrical guide sleeve must still therefore likewise be rather large as well.